In a cellular communication with carrier aggregation (CA) enabled, multiple bands are active at the same time (single mode pass band plus CA mode counter band). In such a carrier aggregation mode, two or more signal paths containing filters or duplexers are circuited in parallel to increase the data rate. No problems arise if in the carrier aggregation mode each band uses a separate antenna. However, in case the two signal paths assigned to two aggregated bands are coupled in parallel to the same antenna mutual loading or blocking is required to avoid power loss.
In receive (RX) CA mode, with signals going from one antenna (AT) to receiver via low noise amplifiers (LNAs), it is important that the signal paths do mutually block and not load each other for each combination of pass band frequency and counter band frequency, because that would otherwise result in power loss and higher insertion loss.
In transmit (TX) CA mode, with signals going from transmitting power amplifiers (PAs) represented by ports P1 and P2, it is important that the signal paths do mutually block and not load each other for each combination of pass band frequency and counter band frequency, because that would otherwise result in power loss and lower output power and lower efficiency.
Here we focus on RX CA mode, but the principles also apply to TX CA mode.
To manage the increasing data traffic, more and more bands assigned to respective frequency ranges are defined. Hence, the number of defined carrier aggregation band combinations increases, too. Front-end modules that can operate within a majority of these bands and band combinations and that are designed according to the conventional approach become big and complex with adverse effects on their performance. Such complex front ends show high insertion losses (IL), and reduced isolation. Further, costs and effort for development increase and the chances for a successful business with these developments get worse.
A standard solution for carrier aggregation makes use of phase shifters which translate the impedance at a given counter frequency band XXX (also called out-of-band (OOB) impedance) into a high impedance like for an open circuit. With two bands to be combined in a CA mode, each band is matched for in-band, and an open circuit is set for OOB. Usually, these two bands for CA mode are coupled to a switch that can be controlled by so-called direct mapping. This means that all throws of the switch can be controlled and activated independently. So it is possible to activate one throw for single band mode or to activate two throws for carrier aggregation mode where two bands are connected in parallel to one antenna terminal or antenna feed to operate simultaneously at the same time. As a consequence, the insertion loss in single band operation mode increases due to the loss of the phase shifter. This is because L and C components that are used have a limited quality factor Q. And in carrier aggregation mode, the insertion loss increases also due to the loading of the counter band in parallel. That is because the magnitude of the OOB reflection coefficient F of the counter band—at the combining reference plane, at the switch—is not an ideal open but finite due to the limited Γ (reflection coefficient) of the filter and the limited Q (quality factor) of the phase shifter. Another disadvantage is that this solution is more or less limited to two parallel bands.
FIG. 1 shows a block diagram of a circuit that allows carrier aggregation mode as well as single mode operation in one of the two bands. The input of a switch SW is connected to an antenna terminal AT. The first output of the switch SW is connected to a first signal path. In the signal path, a phase shifter PS1 is arranged as well as filter means FM1, both circuited in series. The phase shifter PS1 is adapted to provide a high impedance state for signals within the band of a second signal path. The second signal path is connected with a second output of the switch SW and comprises a second phase shifter PS2 and a second filter means FM2. The second phase shifter PS2 is adapted to set a high impedance state for signals within the first band. Terminals providing the respective signal for further operation are referenced by P1, P2.
Another kind of switchplexing within the carrier aggregation mode is shown in FIG. 2. Instead of phase shifters, a series resonator R1, R2 is used to realize an open circuit that is a high impedance state at OOB that means for counter band frequencies. FIG. 2 shows such a circuit where the phase shifters PS from FIG. 1 are substituted by respective resonators R1, R2.
A third known solution for carrier aggregation mode is direct multiplexing shown in FIG. 3. In this case, no switch is involved, the two signal paths each comprising filter means FM are directly connected via a matching and combining network MN. The matching and combining network MN provides a proper in-band matching and good separation for OOB/counter band frequencies, thereby making use of resonance with the out-of-band capacitance of the filter input admittance. This solution is practically also limited to a parallel circuit of only two signal paths operating in two different bands.
When constructing a front-end module for a multitude of different bands that additionally allows carrier aggregation modes for a set of defined band combinations a switch is required having a respective number of throws. Usually more than ten throws are needed to allow a roaming in different regions of the world where different band combinations are in use.
FIG. 4 shows an exemplary block circuit of such a known module. The figure shows an antenna terminal AT connected to a diplexer DIP. The two outputs of which are connected to a switch SW respectively. The upper switch SW shown in the figure is an SP12T, this means the switch SW can couple the switch input and thus, the diplexer with a desired one or a desired combination of the twelve switch outputs. To the outputs, different signal paths operating in different bands are connected. Part of the signal paths comprise filter means, part of the signal paths comprise phase shifters for those bands that are selectable for defined carrier aggregation modes. The second output of the diplexer DIP is connected to a second switch SW′ which is in this embodiment an SP7T switch having seven throws. To this switch, a respective number of signal paths assigned to different bands is connected.
A front-end module like that depicted in FIG. 4 gives rise to different problems. First, in a switch of eight or more switch throws, a crossover point occurs: the insertion loss increases more and more with the number of throws while matching and isolation deteriorates due to coupling between the signal paths or the routing lines for the signal paths respectively. Most of coupling arises around the switch area. Furthermore, the linearity of the switch gets worse for high throw count.
Second, with all bands circuited in parallel via the switch, it becomes more difficult to address all different carrier aggregation band combinations with phase shifters. The design becomes complex or nearly impossible, will be big in size and provides too high insertion losses. Third, with so many bands and carrier aggregation band combinations, it is not attractive for the business of a manufacturer to integrate all the bands and band combinations in one module. Hence, it would be more economical to have a smaller solution which at best should be designed in a modular way and may be extended by plug and play of further parts, if required.